Retrievable multi-pressure cycle firing head

ABSTRACT

A multi-cycle pressure operated firing head for use in a subterranean well. In a described embodiment, a pressure actuated firing head for detonating a perforating gun in a subterranean well includes a firing mechanism and a member which displaces incrementally in response to alternating pressure increases and decreases applied to the firing head. The firing mechanism fires when the member displaces a predetermined distance. In another embodiment, a firing head includes a firing mechanism and a displacement mechanism which displaces a member of the firing head in response to a decrease in a pressure level proximate the firing head in the well. The firing mechanism fires when the member is displaced by the displacement mechanism.

BACKGROUND

[0001] The present invention relates generally to operations performed and equipment utilized in conjunction with subterranean wells and, in an embodiment described below, more particularly relates to a multi-pressure cycle firing head for perforating guns.

[0002] Pressure actuated firing heads are used frequently to initiate detonation of shaped charges in perforating guns. Typically, a pressure operated firing head is set up to actuate at a particular predetermined pressure level in a well. When the pressure level is reached, the firing head is actuated and initiates detonation of the perforating gun shaped charges, either immediately or after a time delay.

[0003] However, in some situations it is not desirable for a pressure actuated firing head to operate in this manner. For example, it may be necessary to pressure test a packer or other equipment prior to detonating the perforating guns. It may not be feasible to exceed the pressure level used in the pressure test at the time it is desired to detonate the perforating guns.

[0004] As another example, the firing head may experience multiple pressure increases in the well prior to the time at which it is desired to actuate the firing head. This situation could occur due to a swabbing effect when the guns and firing head are conveyed into the well, when the guns are positioned in the well while other operations are being performed, etc.

[0005] Therefore, it may be seen that it would be beneficial to provide a pressure actuated firing head which can experience multiple pressure applications prior to firing. In addition, or alternatively, other benefits, such as simplicity of design, elimination of pressurized gas chambers, economy of manufacture, convenience of operation, etc., could be provided in a pressure actuated firing head.

SUMMARY

[0006] In carrying out the principles of the present invention, in accordance with an embodiment thereof, a multi-cycle pressure actuated firing head is provided.

[0007] In one aspect of the invention, a method of firing a firing head to detonate a perforating gun in a subterranean well is provided. The method includes the steps of: conveying the firing head into the well; then alternately increasing and decreasing a pressure level in the well proximate the firing head multiple times; and firing the firing head in response to performance of a predetermined number of the pressure level alternating steps.

[0008] In another aspect of the invention, a pressure actuated firing head for detonating a perforating gun in a subterranean well is provided. The firing head includes a firing mechanism and a member which displaces incrementally in response to alternating pressure increases and decreases applied to the firing head. The firing mechanism fires when the member displaces a predetermined distance.

[0009] In a further aspect of the invention, another pressure actuated firing head for detonating a perforating gun in a subterranean well is provided. The firing head includes a firing mechanism and a displacement mechanism which displaces a member of the firing head in response to a decrease in a pressure level proximate the firing head in the well. The firing mechanism fires when the member is displaced by the displacement mechanism.

[0010] These and other features, advantages, benefits and objects of the present invention will become apparent to one of ordinary skill in the art upon careful consideration of the detailed description of representative embodiments of the invention hereinbelow and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 is a schematic cross-sectional view of a first method embodying principles of the present invention;

[0012]FIG. 2 is a schematic cross-sectional view of a second method embodying principles of the present invention;

[0013] FIGS. 3A-D are cross-sectional views of successive axial portions of a first apparatus embodying principles of the present invention;

[0014] FIGS. 4A-E are cross-sectional views of successive axial portions of a second apparatus embodying principles of the present invention;

[0015] FIGS. 5A-C are schematic cross-sectional views of successive axial portions of a third apparatus embodying principles of the present invention; and

[0016]FIGS. 6A & B are cross-sectional views of successive axial portions of a fourth apparatus embodying principles of the present invention

DETAILED DESCRIPTION

[0017] Representatively illustrated in FIG. 1 is a method 10 which embodies principles of the present invention. In the following description of the method 10 and other apparatus and methods described herein, directional terms, such as “above”, “below”, “upper”, “lower”, etc., are used only for convenience in referring to the accompanying drawings. Additionally, it is to be understood that the various embodiments of the present invention described herein may be utilized in various orientations, such as inclined, inverted, horizontal, vertical, etc., and in various configurations, without departing from the principles of the present invention.

[0018] In the method 10, a tubular string 12 is conveyed into a wellbore 14. The tubular string 12 includes a packer 16, a valve 18, a firing head 20 and a perforating gun 22. The firing head 20 is used to detonate shaped charges (not visible in FIG. 1) in the perforating gun 22, in order to form perforations 24 extending outwardly from the wellbore 14.

[0019] It should be clearly understood that the method 10, and other methods and apparatus described herein, are used merely as examples of applications of the principles of the invention, in order that a person skilled in the art will fully understand how to make and use the invention. However, the principles of the invention are not limited to the specific details of the methods and apparatus described herein. For example, the tubular string 12 in the method 10 could include more, less or different elements instead of those depicted in FIG. 1, these elements could be conveyed together or separately into the wellbore 14, the elements could be differently configured or arranged relative to each other, or relative to the wellbore 14, etc.

[0020] While the firing head 20 and the perforating gun 22 are in the wellbore 14, the firing head may be subjected to a number of increases and decreases in the pressure level proximate the firing head. For example, increases and decreases in the pressure level may be due to circulating fluids through the valve 18, setting the packer 16, pressure testing the packer, etc. However, the firing head 20 is a pressure actuated firing head, i.e., the firing head responds to pressure in the wellbore 14 to fire the gun 22. It would be very hazardous and/or costly for the firing head 20 to fire at any time other than the appropriate time.

[0021] One of the problems solved by the present invention is how to prevent the firing head 20 from firing the gun 22 prematurely. Another of the problems solved by the present invention is how to actuate the firing head 20 at the appropriate time. Yet another problem solved by the present invention is how to permit the firing head 20 to be retrieved from the wellbore 14 without firing the gun 22.

[0022] These problems are solved by providing firing heads (several embodiments of which are described below) which respond to the pressure level in the wellbore 14 in unique ways. One firing head responds to alternating increases and decreases in the pressure level. The firing head fires only when a predetermined number of pressure increases and decreases are performed. The firing head can be safely retrieved from a well if the predetermined number of pressure increases and decreases has not yet been performed.

[0023] Another firing head responds to a decrease in the pressure level only after a predetermined increase in the pressure level has been experienced. The firing head can be safely retrieved from the well if the predetermined increase in the pressure level has not been experienced. If the predetermined increase in the pressure level has been experienced, or if it is not known whether the predetermined increase in the pressure level has been experienced, the firing head can be safely retrieved from the well by separating it from the perforating gun prior to retrieving the firing head.

[0024] Referring additionally now to FIG. 2, another method 30 embodying principles of the invention is representatively illustrated. Elements of the method 30 which are similar to those previously described are indicated in FIG. 2 using the same reference numbers. The method 30 demonstrates additional circumstances in which the benefits of the invention may be achieved.

[0025] In the method 30, the firing head 20 and perforating gun 22 are conveyed into the wellbore 14 separate from a tubular string 32 which includes the packer 16 and valve 18. For example, the firing head 20 and perforating gun 22 could be conveyed using a wireline, coiled tubing string, a production tubing string, etc. The firing head 20 and gun 22 are anchored in the wellbore 14 using a gun hanger 34 set in the wellbore 14.

[0026] Alternatively, the firing head 20 could be conveyed into the wellbore 14 separate from the gun 22. In this manner, the gun 22 could be positioned in the wellbore 14 prior to conveying the firing head 20 into the wellbore and connecting the firing head to the gun, so that pressure increases and decreases in the well prior to conveying the firing head therein would not actuate the firing head. For example, the gun 22 could be positioned in the well during pressure tests of casing or liner 36 lining the wellbore 14, without the risk of firing the firing head 20.

[0027] A connection between the firing head 20 and the gun 22 may be provided by a conventional detonation transfer connection 38 of the type known to those skilled in the art. The connection 38 is a releasable connection which transfers detonation from the firing head 20 to the gun 22. Thus, the firing head 20 may be connected to the gun 22 after the gun has been positioned in the wellbore 14, and the firing head may be separated from the gun after their connection to each other, whether or not the firing head and gun were connected to each other when they were initially conveyed into the wellbore.

[0028] Referring additionally to FIGS. 3A-D, a pressure actuated firing head 40 embodying principles of the invention is representatively illustrated. The firing head 40 may be used for the firing head 20 in the methods 10, 30 described above. The firing head 40 may also be used in other methods, without departing from the principles of the invention.

[0029] The firing head 40 includes a firing mechanism 42 and a displacement mechanism 44. The firing mechanism 42 includes a firing piston 46, a firing pin 48 and a release mechanism 28 including a series of circumferentially distributed release members or collets 50 attached to the piston. Note that release members other than the collets 50 could be used, for example, a C-ring or snap ring, keys, dogs, etc., could be used in place of the collets.

[0030] Wellbore pressure is applied to an upper portion of the piston 46 via one or more openings 52 formed through an outer housing assembly 54 of the firing head 40. A lower portion of the piston 46 is exposed to pressure in an atmospheric chamber 56. Thus, when the firing head 40 is subjected to pressure in a wellbore greater than atmospheric pressure, the piston 46 is biased downwardly by the wellbore pressure.

[0031] The collets 50 initially prevent downward displacement of the piston 46. Upper radially enlarged ends 58 of the collets 50 are received in an annular recess 60 formed in the housing assembly 54. A member or rod 62 outwardly supports the collets 50, so that the upper ends 58 remain engaged with the recess 60, thereby preventing downward displacement of the piston 46.

[0032] Note that, if the rod 62 is displaced upward a sufficient distance relative to the collets 50, the rod will eventually fail to outwardly support the collets. This will permit the upper ends 58 of the collets 50 to disengage from the recess 6o, thereby permitting the piston 46 to displace downwardly and impact the firing pin 48. This impact will shear a pin 64 retaining the firing pin 48 in position and the impact will be transmitted to a detonation initiator 66. If the firing head 40 is operatively connected to a perforating gun when the initiator 66 is impacted, the gun will be detonated in a manner known to those skilled in the art.

[0033] Therefore, it will be readily appreciated that upward displacement of the rod 62 is used to fire the firing head 40. This displacement of the rod 62 is controlled by the displacement mechanism 44. However, it will also be appreciated that displacement of the rod 62 will not result in firing of the firing head 40, unless the firing head is subjected to wellbore pressure greater than atmospheric pressure to downwardly bias the piston 46 and, thus, the firing head is inherently prevented from firing at the surface.

[0034] The displacement mechanism 44 includes a piston 68, a biasing device 70 and two engagement devices 72, 74. As depicted in the drawings, the biasing device 70 is a pressurized gas chamber containing, for example, nitrogen at an elevated pressure. Other biasing devices, such as springs, etc., could be used in place of the chamber 70, without departing from the principles of the invention.

[0035] The engagement devices 72, 74 are depicted in the drawings as including gripping members or segmented slips 76 received in tapered bores 78, the slips being biased toward reduced diameter portions of the bores by springs 80. The rod 62 is received within the slips 76, each of which operates to prevent downward displacement of the rod, but to permit upward displacement of the rod relative thereto. Thus, each of the engagement devices 72, 74 permits only upward displacement of the rod 62. It should be clearly understood that other types of engagement devices could be used in the firing head 40, in keeping with the principles of the invention.

[0036] The upper engagement device 72 is connected to the piston 68 and displaces therewith. The other engagement device 74 is attached to the housing assembly 54. If the piston 68 displaces upward, the upper engagement device 72 will cause the rod 62 to be displaced upward with the piston (the lower engagement device 74 permitting such upward displacement of the rod). If the piston 68 displaces downward, the lower engagement device 74 will prevent the rod 62 from displacing downward with the piston (the upper engagement device 72 permitting upward displacement of the rod relative to the piston).

[0037] Therefore, it will be readily appreciated that, by reciprocating the piston 68 alternately upward and downward relative to the housing assembly 54, the rod 62 can be made to displace incrementally upward with every upward displacement of the piston. When the rod 62 has been displaced incrementally upward a sufficient number of times, the rod will displace a corresponding sufficient distance so that it no longer outwardly supports the collets 50, as described above.

[0038] An upper portion of the piston 68 is exposed to pressure in the gas chamber 70. A lower portion of the piston 68 is exposed to wellbore pressure via one or more openings 82 formed through the housing assembly 54. When wellbore pressure exceeds pressure in the gas chamber 70, the piston 68 will displace upwardly. The piston 68 has been upwardly displaced as viewed in FIG. 3B.

[0039] When pressure in the gas chamber 70 exceeds wellbore pressure, the piston 68 will displace downwardly. Thus, to reciprocate the piston 68 upwardly and downwardly, pressure in the wellbore proximate the firing head 40 should be alternately increased and decreased, respectively above and below the pressure in the gas chamber 70.

[0040] Preferably, although not necessarily, the pressure in the gas chamber 70 is set so that it is greater than the wellbore pressure at any time up until it is desired to fire the firing head 40. For example, if prior to the time it is desired to fire the firing head 40 wellbore pressure is expected to be as great as 10,000 psi proximate the firing head, the gas chamber pressure may be set at 11,000 or 12,000 psi. In this manner, upward displacement of the piston 68 may be prevented until it is desired to fire the firing head 40, at which time the wellbore pressure may be alternately increased above the gas chamber pressure and decreased below the gas chamber pressure.

[0041] In some situations it may not be feasible to set the gas chamber pressure greater than the maximum wellbore pressure up until it is desired to fire the firing head 40. For example, if the maximum wellbore pressure is due to a packer pressure test, it may not be desirable to subsequently exceed that pressure in the wellbore at the firing head 40. In these situations, the gas chamber 70 pressure may be set less than the maximum wellbore pressure.

[0042] A shear pin 84 may be used to prevent upward displacement of the rod 62 until a predetermined wellbore pressure is reached. The shear pin 84 passes through the rod 62 and an annular collar 86 encircling the rod and abutting a lower end of the lower engagement device 74. When the predetermined wellbore pressure is reached, the pin 84 shears, thereby permitting upward displacement of the rod 62.

[0043] The shear pin 84 may be installed in any of several axially spaced apart holes 92 provided in the rod 62. By installing the pin 84 in a more upwardly positioned one of the holes 92, the rod 62 is more downwardly positioned initially relative to the housing assembly 54, and so it takes a greater number of pressure cycles to upwardly displace the rod a sufficient distance to fire the firing head 40. Similarly, by installing the pin 84 in a more downwardly positioned one of the holes 92, the rod 62 is more upwardly positioned initially relative to the housing assembly 54, and so it takes a lesser number of pressure cycles to upwardly displace the rod a sufficient distance to fire the firing head 40. The rod 62 can also be made longer or shorter, or initially positioned more upwardly or downwardly in the firing head 40 by other means, to achieve a desired number of pressure cycles required to fire the firing head.

[0044] The shear pin 84 may be set to shear at a wellbore pressure exceeding the gas chamber 70 pressure by a certain amount, for example, 1,000 psi. In this manner, reciprocating displacement of the piston 68 while the firing head 40 is conveyed into the well, or when other pressure increases and decreases are experienced, may be prevented in those situations where the gas chamber 70 pressure is set lower than the maximum wellbore pressure to be experienced at the firing head 40 prior to the time it is desired to fire the firing head.

[0045] For example, if the gas chamber 70 pressure is set below a packer test pressure, the shear pin 84 could be set to shear at a wellbore pressure greater than the gas chamber pressure but less than the packer test pressure. The first upward displacement of the piston 68 would occur when the shear pin 84 shears (for example, when the packer is tested). Subsequent decreases and increases in wellbore pressure, respectively below and above the gas chamber 70 pressure will cause the piston 68 to reciprocate downwardly and upwardly in the firing head 40, eventually displacing the rod 62 a sufficient distance to fire the firing head.

[0046] Of course, it is not necessary for the shear pin 84 to be set to shear at a wellbore pressure related to any particular event, such as a packer pressure test. For example, the shear pin 84 could be set to shear at a wellbore pressure corresponding to an elevated hydrostatic pressure to be experienced by the firing head 40 during or after conveyance into the well, in order to prevent reciprocating displacement of the piston 68 prior to the firing head experiencing that elevated hydrostatic pressure.

[0047] It should also be noted that, until the shear pin 84 shears, the firing head 40 may be safely retrieved from the well without firing. In addition, until a wellbore pressure greater than the gas chamber 70 pressure has been experienced at the firing head 40, the firing head may also be retrieved safely from the well. Furthermore, until a sufficient number of alternating wellbore pressure increases and decreases have been experienced at the firing head 40, the firing head may be safely retrieved from the well. And, since the firing mechanism 42 actually fires only in response to a pressure increase (resulting in upward displacement of the rod 62), the firing head 40 may be safely retrieved from the well as long as a pressure increase is not experienced by the firing head during such retrieval.

[0048] A conventional fishing neck 88 is provided at an upper end of the firing head 40 as depicted in the drawings, for conveyance of the firing head into the well (as in the method 30) or retrieval of the firing head from the well, either connected to, or separate from, a perforating gun. Alternatively, the firing head 40 may be provided with a connection permitting it to be interconnected in a tubing string (as in the method 10).

[0049] A conventional threaded seal bore connection 90 is provided at a lower end of the firing head 40 as depicted in the drawings, for connection of the firing head to a perforating gun, or to a detonation transfer connection. However, it should be understood that any types of connections may be provided on the firing head 40, the firing head may be connected above or below a perforating gun, and the firing head may be conveyed into the well, or retrieved therefrom, by any method, in keeping with the principles of the invention. These considerations apply equally well to the other firing heads described below.

[0050] Referring additionally now to FIGS. 4A-E, another firing head 100 embodying principles of the invention is representatively illustrated. The firing head 100 may be used for the firing head 20 in the methods 10 and 30, or in any other methods. The firing head 100 is similar in many respects to the firing head 40 described above, and elements shown in FIGS. 4A-E which are similar to those previously described are indicated using the same reference numbers. Note that the firing mechanism 42 of the firing head 100 is substantially identical to that of the firing head 40 (the firing mechanism firing the firing head by upwardly displacing the rod 62).

[0051] However the firing head 100 has a different displacement mechanism 102 for displacing the rod 62. The displacement mechanism 102 includes upper and lower pistons 104, 106, with a biasing device or pressurized gas chamber 108 between the pistons. An elongated rod-like member 110 connects the pistons 104, 106 to each other, so that they displace with each other. This assembly (pistons 104, 106 and rod member 110) is connected to the upper engagement device 72, so that the rod 62 displaces upwardly with the assembly.

[0052] An upper portion of the upper piston 104 is exposed to wellbore pressure via one or more openings 112 formed through a housing assembly 114. A lower portion of the lower piston 106 is exposed to wellbore pressure via one or more openings 116 formed through the housing assembly 114. Because the upper piston 104 has a greater piston area than the lower piston 106, wellbore pressure will bias the combined pistons and rod member 10 assembly downwardly.

[0053] Pressure in the chamber 108 is exposed to a lower portion of the upper piston 104, and to an upper portion of the lower piston 106. Since the upper piston 104 has a greater piston area than the lower piston 106, pressure in the chamber 108 will bias the combined pistons and rod member 110 assembly upwardly. By alternately increasing and decreasing the wellbore pressure, respectively above and below the gas chamber 108 pressure, the rod 62 may be incrementally displaced upwardly a sufficient distance to fire the firing head 100, similar to the manner in which the firing head 40 may be operated as described above.

[0054] However, the combined pistons 104, 106 and rod member 110 assembly displace upwardly in response to a decrease in the wellbore pressure, instead of in response to an increase in wellbore pressure. Thus, the gas chamber 108 may pressurized so that it is at a pressure greater than wellbore pressure experienced at the firing head 100 while it is being conveyed into the well. In this manner, when it is desired to fire the firing head, wellbore pressure at the firing head 100 may be increased above the gas chamber 108 pressure to downwardly displace the pistons 104, 106, rod member 110 and upper engagement device 72, and then wellbore pressure at the firing head may be decreased to upwardly displace those elements along with the rod 62.

[0055] The shear pin 84 may be set so that a certain decreased wellbore pressure at the firing head 100 is required to upwardly displace the rod 62, as in the firing head 40 described above. In addition, the lower end of the rod 62 may be positioned within the collets 50 a distance which corresponds to only a single upward displacement of the rod 62. In this manner, the firing head 100 may be fired upon the first decrease in wellbore pressure at the firing head 100 after the wellbore pressure exceeds the pressure in the gas chamber 108. This decrease in wellbore pressure at the firing head 100 may be achieved, for example, by using gas lift equipment to displace fluid out of the wellbore.

[0056] Referring additionally now to FIGS. 5A-C, another firing head 120 embodying principles of the invention is representatively and schematically illustrated. The firing head 120 may be used for the firing head 20 in the methods 10 and 30, or in any other methods. The firing head 120 is similar in many respects to the firing head too described above, and elements which are similar to those previously described are indicated in FIGS. 5A-C using the same reference numbers.

[0057] The firing head 120 is distinguished from the firing head 100 by two main differences. First, an upper piston 122 of the firing head 120 has a smaller piston area than a lower piston 124. Thus, wellbore pressure at the firing head 120 biases the pistons 122, 124 and a rod member 126 connecting the pistons upwardly, instead of downwardly as in the firing head 100. Accordingly, the firing head 120 fires upon an increase in wellbore pressure at the firing head, 20 similar to the firing head 40.

[0058] Second, a biasing device 128 of the firing head 120 biases the pistons 122, 124 and rod member 110 downwardly, instead of upwardly as in the firing head 100. The biasing device 128 is one or more springs, such as Bellville washers, of the type known to those skilled in the art.

[0059] Since the rod 62 may be displaced incrementally upward by alternately increasing and decreasing the wellbore pressure at the firing head 120, so that the biasing force exerted by the wellbore pressure on the pistons 122, 124 is respectively greater than and less than the biasing force exerted by the springs 128, the firing head 120 may be operated in the same manners as those described above for the firing head 40. For example, the springs 128 may be configured so that they exert a biasing force greater than that exerted by the wellbore pressure on the pistons 122, 124 at any time up until it is desired to fire the firing head 120. When it is desired to fire the firing head 120, the wellbore pressure may be alternately increased and decreased to incrementally displace the rod 62 upward.

[0060] As another example, the springs 128 may be configured so that they exert a biasing force less than that exerted by the maximum wellbore pressure on the pistons 122, 124 prior to the time it is desired to fire the firing head 120, but greater than that exerted by the wellbore pressure on the pistons during conveyance of the firing head into the well. This will prevent the pistons 122, 124 from reciprocating upward and downward during conveyance into the well, while not requiring the wellbore pressure to be increased greater than the maximum wellbore pressure on the pistons 122, 124, when it is desired to fire the firing head 120.

[0061] Referring additionally now to FIGS. 6A & B, another firing head 130 embodying principles of the invention is representatively and schematically illustrated. The firing head 130 may be used for the firing head 20 in the methods 10, 30, or in other methods. The firing head is similar in many respects to the firing head 120 described above, and elements which are similar to those previously described are indicated in FIGS. 6A & B using the same reference numbers.

[0062] One significant difference between the firing heads 120, 130 is that multiple biasing devices 128 are used in the firing head 130. An upper one of the biasing devices 128 exerts a downwardly directed force on the upper piston 122. A lower one of the biasing devices 128 exerts a downwardly directed biasing force on the lower piston 124.

[0063] Another significant difference is that the firing head 130 includes a member 132 outwardly supporting the collets 50 which is downwardly, instead of upwardly, displaced in order to fire the firing head. The member 132 is initially positioned in the upper ends 58 of the collets and secured with a shear pin 134. When a sufficiently great downwardly directed force is applied to an upper end of the member 132, the pin 134 shears, permitting the member to displace downwardly. This downward displacement of the member 132 causes the upper ends 58 of the collets 50 to disengage from the recess 60, permitting the collets and piston 46 to displace downwardly and fire the firing head 130.

[0064] A further difference is that the lower piston 124 has an engagement device 136 attached thereto which is oppositely configured as compared to the engagement devices 72, 74 described above. That is, the engagement device 136 permits downward displacement of a rod member 138 therethrough, but prevents upward displacement of the rod member.

[0065] Thus, when the pistons 122, 124 displace upwardly, the rod member 138 extends outwardly and downwardly from the lower piston 124, rather than displacing upwardly with the lower piston. Conversely, when the pistons 122, 124 displace downwardly, the rod member 138 displaces downwardly with the lower piston 124, the engagement device 136 preventing upward displacement of the rod member relative to the lower piston.

[0066] Actuation of the firing head 130 is as follows: When wellbore pressure at the firing head 130 exerts an upwardly biasing force on the pistons 122, 124 greater than a downwardly biasing force exerted by the biasing devices 128, the pistons and rod member 126 displace upwardly. A radially enlarged lower end 140 of the rod member 138 prevents it from displacing upwardly with the lower piston 124, and the engagement device 136 permits the lower piston to displace upwardly relative to the rod member. Thus, the rod member 138 extends further outwardly and downwardly from the lower piston 124 as it displaces upwardly.

[0067] When wellbore pressure at the firing head 130 is subsequently decreased, the downwardly biasing force exerted by the biasing devices 128 eventually exceeds the upwardly biasing force exerted on the pistons 122, 124 by the wellbore pressure, and the pistons displace downwardly. The engagement device 136 prevents downward displacement of the lower piston 124 relative to the rod member 138, and so the rod member displaces downwardly with the lower piston.

[0068] As the rod member 138 displaces downwardly, the lower end 140 of the rod member contacts an upper end of the member 132 and applies the downwardly directed biasing force to the member. When the downwardly directed biasing force is sufficiently great, the pin 134 shears, permitting the member 132 to displace downwardly. Downward displacement of the member 132 causes the upper ends 58 of the collets 50 to disengage from the recess 60, thereby firing the firing head 130 as described above. Note that the shear pin 134 may be sized so that a certain predetermined decreased wellbore pressure must be reached before the pin will shear to release the member 132.

[0069] Of course, a person skilled in the art would, upon a careful consideration of the above description of representative embodiments of the invention, readily appreciate that many modifications, additions, substitutions, deletions, and other changes may be made to these specific embodiments, and such changes are contemplated by the principles of the present invention. Accordingly, the foregoing detailed description is to be clearly understood as being given by way of illustration and example only, the spirit and scope of the present invention being limited solely by the appended claims and their equivalents. 

What is claimed is:
 1. A method of firing a firing head to detonate a perforating gun in a subterranean well, the method comprising the steps of: conveying the firing head into the well; then alternately increasing and decreasing a pressure level in the well proximate the firing head multiple times; and firing the firing head in response to performance of a predetermined number of the pressure level alternating steps.
 2. The method according to claim 1, wherein the pressure level alternating step further comprises increasing the pressure level above a predetermined pressure multiple times, and wherein the firing step further comprises firing the firing head in response to performance of a predetermined number of the pressure level increasing steps.
 3. The method according to claim 1, wherein the pressure level alternating step further comprises decreasing the pressure level below a predetermined pressure after increasing the pressure level, and wherein the firing step further comprises firing the firing head in response to performance of the pressure level decreasing step.
 4. The method according to claim 1, wherein the conveying step further comprises conveying the firing head into the well attached to the perforating gun.
 5. The method according to claim 4, wherein the pressure level alternating steps are performed while the firing head is attached to the perforating gun in the well.
 6. The method according to claim 1, wherein the conveying step further comprises conveying the firing head into the well separate from the perforating gun.
 7. The method according to claim 1, further comprising the step of operatively attaching the firing head to the perforating gun after the conveying step and prior to the firing step.
 8. The method according to claim 1, further comprising the step of incrementally displacing a member of the firing head in response to each of the pressure level alternating steps.
 9. The method according to claim 8, wherein the firing step further comprises firing the firing head when the member has displaced a predetermined total distance.
 10. The method according to claim 8, further comprising the step of releasing a firing pin of the firing head to displace in response to a biasing force exerted by the pressure level, when the member has displaced a predetermined total distance.
 11. The method according to claim 8, wherein the incrementally displacing step further comprises incrementally displacing the member in response to a selected one of the steps of increasing and decreasing the pressure level, and retaining the member against displacement in response to the other one of the steps of increasing and decreasing the pressure level.
 12. The method according to claim 1, further comprising the step of displacing a piston of the firing head in alternating opposite directions against a force exerted by a biasing device, the displacing step being performed in response to the pressure level alternating step.
 13. A pressure actuated firing head for detonating a perforating gun in a subterranean well, the firing head comprising: a firing mechanism; and a member which displaces incrementally in response to alternating pressure increases and decreases applied to the firing head, the firing mechanism firing when the member displaces a predetermined distance.
 14. The firing head according to claim 13, wherein the member displaces in response to each of the pressure increases.
 15. The firing head according to claim 13, wherein the member remains motionless during each of the pressure decreases.
 16. The firing head according to claim 13, further comprising a displacement mechanism operative to incrementally displace the member in response to the pressure increases and decreases, the displacement mechanism including a piston and a biasing device.
 17. The firing head according to claim 16, wherein during the pressure increases the piston displaces in a first direction against a force exerted by the biasing device, and wherein during the pressure decreases the biasing device displaces the piston in a second direction opposite to the first direction.
 18. The firing head according to claim 16, wherein the displacement mechanism further includes a first engagement device which engages the member, so that the member displaces with the piston when the piston displaces in a first direction, and a second engagement device which engages the member, so that the piston displaces relative to the member when the piston displaces in a second direction opposite to the first direction.
 19. The firing head according to claim 18, wherein each of the first and second engagement devices includes at least one grip member which grips the member and prevents displacement of the member in the second direction.
 20. The firing head according to claim 18, wherein the first engagement device is attached to, and displaces with, the piston, and wherein the piston displaces reciprocably relative to the second engagement device in response to the pressure increases and decreases.
 21. The firing head according to claim 16, wherein the biasing device is at least one spring.
 22. The firing head according to claim 16, wherein the biasing device is at least one chamber having pressurized gas therein.
 23. The firing head according to claim 13, wherein the firing mechanism includes a release mechanism and a firing pin, the release mechanism permitting the firing pin to displace when the member has displaced the predetermined distance.
 24. The firing head according to claim 23, wherein the release mechanism includes at least one collet supported initially by the member, the collet becoming unsupported and thereby permitting the firing pin to displace when the member has displaced the predetermined distance.
 25. The firing head according to claim 23, wherein the firing pin displaces in response to pressure proximate the firing head when the member has displaced the predetermined distance.
 26. A pressure actuated firing head for detonating a perforating gun in a subterranean well, the firing head comprising: a firing mechanism; and a displacement mechanism which displaces a member of the firing head in response to a decrease in a pressure level proximate the firing head in the well, the firing mechanism firing when the member is displaced by the displacement mechanism.
 27. The firing head according to claim 26, wherein the displacement mechanism displaces the member in response to only a single decrease in the pressure level in the well.
 28. The firing head according to claim 26, wherein the displacement mechanism displaces the member in response to the decrease in the pressure level only if the decrease in the pressure level is preceded by an increase in the pressure level.
 29. The firing head according to claim 28, wherein the displacement mechanism displaces the member in response to the decrease in the pressure level only if the decrease in the pressure level is preceded by the increase in the pressure level above a first predetermined pressure.
 30. The firing head according to claim 29, wherein the displacement mechanism displaces the member in response to the decrease in the pressure level only if the decrease in the pressure level decreases the pressure level below a second predetermined pressure.
 31. The firing head according to claim 26, wherein the member displaces only in response to the decrease in the pressure level.
 32. The firing head according to claim 26, wherein the member remains motionless during pressure increases in the well.
 33. The firing head according to claim 26, wherein the displacement mechanism includes a piston and a biasing device.
 34. The firing head according to claim 33, wherein during increases in the pressure level, the piston displaces in a first direction against a force exerted by the biasing device, and wherein during the decrease in the pressure level, the biasing device displaces the piston in a second direction opposite to the first direction.
 35. The firing head according to claim 33, wherein the displacement mechanism further includes an engagement device which permits the member to displace relative to the piston when the piston displaces in a first direction, and which engages the member so that the member displaces with the piston when the piston displaces in a second direction opposite to the first direction.
 36. The firing head according to claim 35, wherein the engagement device includes at least one grip member which grips the member and prevents displacement of the member in the first direction relative to the piston.
 37. The firing head according to claim 35, wherein the engagement device is attached to, and displaces with, the piston.
 38. The firing head according to claim 33, wherein the biasing device is at least one spring.
 39. The firing head according to claim 33, wherein the biasing device is at least one chamber having pressurized gas therein.
 40. The firing head according to claim 26, wherein the firing mechanism includes a release mechanism and a firing pin, the release mechanism permitting the firing pin to displace when the member is displaced by the displacement mechanism.
 41. The firing head according to claim 40, wherein the release mechanism includes at least one collet supported initially by the member, the collet becoming unsupported and thereby permitting the firing pin to displace when the member is displaced by the displacement mechanism.
 42. The firing head according to claim 40, wherein the firing pin displaces in response to the pressure level proximate the firing head when the member is displaced by the displacement mechanism. 